1. Field of the Invention
The present invention relates to a method of firing ceramic honeycomb structural bodies, in which cordierite raw materials including at least raw kaolin is extruded to obtain a honeycomb structural formed body and the thus obtained honeycomb structural body is fired.
2. Description of Related Art
Generally, as a method of firing ceramic honeycomb structural bodies, in which cordierite raw materials including talc, kaolin, alumina and so on is extruded to obtain a honeycomb structural formed body and the thus obtained honeycomb structural body is fired, various firing methods have been known. For example, in Japanese Patent Laid-Open Publication No. 1-249665, in order to prevent a crack generation due to an exothermic reaction in a decomposition temperature range of forming agents, it has been known the technique that a temperature ascending rate till the forming agent decomposition temperature is decreased as compared with that of over the forming agent decomposition temperature. That is to say, in this technique, the firing is performed in such a manner that a temperature ascending rate till 200.degree. C. is set to 80-90.degree. C./hr and a temperature ascending rate of over that temperature is set to 100-120.degree. C./hr.
Moreover, in Japanese Patent Laid-Open Publication No. 2-255576, in order to prevent a deformation of the ceramic honeycomb structural body, a temperature ascending rate is maintained under 60.degree. C. in a temperature range (1100-1180.degree. C.) in which the honeycomb structural body is shrunk by heat. Further, in Japanese Patent Laid-Open Publication No. 5-85856, in order to optimize the properties such as a water absorption rate and a thermal expansion coefficient rate, a temperature ascending rate is maintained (1) under 60.degree. C./hr in a temperature range (1100-1200.degree. C.) in which the honeycomb structural body is shrunk by heat, (2) over 80.degree. C./hr in a solid phase reaction temperature range (1200-1300.degree. C.), and (3) under 60.degree. C./hr in a liquid phase reaction temperature range (1300-keep temperature).
On the other hand, as a carrier constructed by the honeycomb structural body used for purifying an exhaust gas from automobiles, it is necessary to increase a cell density so as to improve a purifying performance. In the honeycomb structural body, if the cell density is increased, a pressure loss is necessarily increased. Therefore, if the cell density is increased, an engine power is decreased. In order to reduce the pressure loss, it is necessary to make a rib thickness thinner. Generally, honeycomb structural bodies having a rib thickness of 6.0-6.6 mil, which are so-called as 6 mil body, are mainly used. However, recently, honeycomb structural bodies having a rib thickness of under 4.6 mil, which are so-called as thin wall body, are used increasingly.
When the thin wall honeycomb structural body having a rib thickness of under 4.6 mil is to be fired according to the known firing methods mentioned above, a crack is liable to be generated during the firing step as compared with the known honeycomb structural body having relatively thick wall thickness. Particularly, in order to obtain the honeycomb structural body having a thin wall thickness, a raw kaolin is used recently as a part of kaolin so as to improve a flowability of raw materials when they are passed through the die. In this case, since a crystallization water is removed from the raw kaolin at a temperature of 400-600.degree. C. and this crystallization water removing reaction is a heat absorbing reaction, a temperature difference is generated in the honeycomb structural body during this heat absorbing reaction, so that a crack is liable to be generated in the honeycomb structural body.